49 



MECHANISM OF THE BONES AND JOINTS. 





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Hyaline 



tion. The transverse striation is still visible as long as the sarcous substance is in large masses, 

 but as soon as it breaks up into small pieces the transverse striation disappears completely 

 (0. Both). 



305. MECHANISM OF THE BONES AND JOINTS. Bones exhibit in the 

 inner architecture of their spongiosa an arrangement of their lamellae and spicules 

 which represents the static result of those forces pressure and traction which act 

 on the developing bone (Structure of Bone, 447). They are so arranged that, 

 with the minimum of material, they afford the greatest resistance as a supporting 

 structure or framework (//. v. Meyer, Culma?in, Jul. Wolf). 



I. The joints permit the freest movements of one bone upon another, [such as exist between 

 the extremities of the bones of the limbs. In other cases, sutures are formed, which, while 

 permitting no movement, allow the contents of the cavity which they surround to enlarge, as 

 in the case of the cranium]. The articular end of a fresh bone is covered with a thin layer or 

 plate of hyaline cartilage, which in virtue of its elasticity moderates any shocks or impulses 

 communicated to the bones. The surface of the articular cartilage is perfectly smooth, and 

 facilitates an easy gliding movement of the one surface upon the other. At the outer boundary 

 line of the cartilage, there is fixed the capsule of the joint, which encloses the articular ends of 

 the bones like a sac. The inner surface of the capsule is lined by a synovial membrane, which 

 secretes the sticky, semi-fluid, synovia, moistening the joint. The outer surface of the capsule 

 is provided at various parts with bands of fibrous tissue, some of which strengthen it, whilst 

 others restrain or limit the movement of the joint. Some osseous processes limit the movements 



of particular joints, e.g., the coronoid process of the ulna, which 

 permits the fore-arm to be flexed on the upper arm only to a 

 certain extent ; the olecranon, which prevents over-extension 

 at the elbow-joint. The joint surfaces are kept in apposition 

 (1) by the adhesion of the synovia-covered smooth articular 

 surface ; (2) by the capsule and its fibrous bands ; and (3) by 

 the elastic tension and contraction of the muscles. 



[Structure of Articular Cartilage. The thin layer of hyaline 

 encrusting cartilage is fixed by an irregular surface upon the 

 curtilage, corresponding surface of the head of the bone (fig. 345). In a 

 vertical section through the articular cartilage of a bone which 

 has been softened in chromic or other suitable acid, we observe 

 that the cartilage cells are flattened near the free surface of the 

 cartilage, and their long axes are parallel to the surface of the 

 joint ; lower down, the cells are arranged in irregular groups, 

 and farther down still, nearer the bone, in columns or rows, 

 whose long axis is in the long axis of the bone. These rows 

 are produced by transverse cleavage of pre-existing cells. In 

 the upper two-thirds or thereby the matrix of the cartilage is 

 hyaline, but in the lower third, near the bone, the matrix is 

 granular and sometimes fibrillated. This is the calcified zone, 

 which is impregnated with lime salts, and sharply defined by 

 a nearly straight line from the hyaline zone above it, and by a 

 very bold wavy line from the osseous head of the bone.] 



Synovial Membrane. Synovial membrane consists of bundles 

 of delicate connective-tissue mixed with elastic tissue, while on 

 its inner surface it is provided with folds, some of which con- 

 tain fat, and others blood-vessels (synovial villi). The inner 

 surface is lined with endothelium. The intra-capsular liga- 

 ments and cartilages are not covered by the synovial membrane, 

 nor are they covered by endothelium. The synovia is a colour- 

 less, stringy, alkaline fluid, with a chemical composition closely 

 allied to that of transudations, with this difference, that it contains much mucin, together with 

 albumin and traces of fat. N Excessive movement diminishes its amount, makes it more inspis- 

 sated, and increases the mucin, but diminishes the salts. 



Joints may be divided into several classes, according to the kind of movement 

 which they permit : 



L Joints with movement around one axis: (a) The Ginglymus, or Hinge- Joint. The one 

 articular surface represents a portion of a cylinder or sphere, to which the other surface is 

 adapted by a corresponding depression, so that, when flexion or extension of the joint takes 

 place, it moves only on one axis of the cylinder or sphere. The joints of the fingers and toes are 

 hinge-joints of this description. Lateral ligaments, which prevent a lateral displacement of 

 the articular surfaces, are always present. 



8 



A 







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Calcified 

 cartilage. 



Bone. 



Fig. 345. 

 Vertical section of articular car- 

 tilage (Stirling). 



